1. Field of the Invention
The present invention relates to a communications system and, more specifically, a communications method for use in a system having devices inserted therein or removed therefrom.
2. Description of the Background Art
Conventionally, various techniques for recovering synchronization in communication among devices on a network have been devised. Furthermore, in recent years, digital devices have become widespread, and networks for digital devices have also been widely available. A typical example of interfaces for connecting digital devices is an interface under IEEE 1394 standards. In a digital network of IEEE 1394, for example, a function of removing and connecting an arbitrary device (plug-and-play) is often used. Even on such a network, if synchronization is lost, a process of recovering synchronization is of great importance.
In data communications using digital devices, the devices have to be synchronized with each other in data transmission and reception. In a data receiving device, if synchronization between a received data signal and a clock signal is not established, it is impossible to correctly perform data identification. In order to establish synchronization, a typical scheme is a scheme (A) of transmitting a data signal together with a clock signal, or a scheme (B) of reproducing a data signal based on a clock signal. In the scheme (A), synchronization on a transmission path is established only at data transmission. The scheme (B) includes a scheme (B1), in which synchronization on a transmission path is established only at data transmission by first transmitting a signal for establishing synchronization and then transmitting a data signal, and a scheme (B2), in which synchronization on a transmission path is always established by transmitting a signal that carries no information (idle signal) even when data transmission is not performed.
With reference to FIG. 8, a data transmitting scheme under P1394b is described below, which is one example of the above scheme (B2). First, each device is in a state of being unconnected (S1). Upon being connected to another device via a transmission path, the device detects connection and exchanges information regarding a data transfer rate with the other device. This makes it possible for the device to determine its data transfer rate and then establish connection (S2). Then, the device transmits a signal for establishing synchronization on the transmission path at the determined data transfer rate. Then, devices at both ends of the transmission path establish synchronization on the transmission path (S3). Then, the devices each perform initialization of a bus, arbitration, and then exchanges of information regarding its own device, thereby establishing synchronization among the devices (S4). During actual data communications, the devices perform arbitration there among as required, and then perform data communications (S5). Also, under P1394b, upon removal or connection of a device on a plug-and-play basis, the state of connection of the devices connected to the network is changed by a bus reset signal to a state of synchronization on the transmission path (S3). After bus reset is completed, a procedure of establishing connection among the devices is again performed through data transmission.
Here, in a network where the plug-and-play function is available, removal or connection of a device may cause noise, for example, which may lead to a loss of synchronization on a part of transmission paths among devices configuring the network. Regarding such a loss of synchronization, the inventor has found a possible occurrence of the following phenomenon in a network using P1394b.
In a communications system where state transitions such as those illustrated in FIG. 8 occur, removal/connection of a device generally causes a change in the state of the devices configuring the network from the device-connected state (S4) or the communications state (S5) to the path-synchronized state (S3). However, if removal or connection of a device causes a loss of synchronization on a part of the transmission paths of the network, the state of devices connected to that transmission path is changed to the path-connected state (S2) (refer to a dotted arrow in FIG. 8). In this case, the devices try to perform a process for establishing synchronization again, but may fail to successfully establish synchronization, thereby causing a stop of the process.
Moreover, in the above case, the malfunctioning transmission path makes it impossible to transmit a bus reset signal. Therefore, restructuring the network cannot be made. Consequently, the network is divided into two networks by the malfunctioning transmission path. Thus, it is impossible to perform communications via the malfunctioning transmission path on the network. In that case, the user conventionally has to perform the following process. First, the user locates the malfunctioning transmission path. Then, in one scheme, the user physically removes and then reconnects the located malfunctioning transmission path. In another scheme, the user resets devices at both ends of the located malfunctioning transmission path.